The present invention relates to closed cell rigid polyurethane or urethane-modified polyisocyanurate foams having improved flammability performance and blown with hydrocarbon blowing agents in combination with a minor amount of water. The invention includes the process used to produce the foams, novel compositions useful in said process and the foams prepared thereby.
Rigid polyurethane foams have many known uses, such as in building materials and thermal insulation. Such foams are known to have excellent flammability performance, outstanding initial and long term thermal insulation and superior structural properties.
Rigid polyurethane foams have conventionally been prepared by reacting appropriate polyisocyanate and isocyanate-reactive compositions in the presence of a suitable blowing agent With regard to blowing agents, chlorofluorocarbons (CFC's) such as trichlorofluoromethane (CFC-11) and dichlorodifluoromethane (CFC-12) have been used most extensively as they have been shown to produce foams having low flammability, good thermal insulation properties and excellent dimensional stability. However, in spite of these advantages, CFC's have fallen into disfavor, as they have been associated with the depletion of ozone in the earth's atmosphere, as well as possible global warming potential. Accordingly, the use of CFC's has been severely restricted.
Hydrochlorofluorocarbons (HCFC's) such as chlorodifluoromethane (HCFC-22), 1-chloro-1,1-difluoroethane (HCFC-142b), and particularly 1,1-dichloro-1-fluoroethane (HCFC-141b) have been considered a viable interim solution. However, HCFC's have also been shown to cause a similar depletion of ozone in the earth's atmosphere and accordingly, their use has also come under scrutiny. In fact, the widespread production and use of HCFC-141b is presently scheduled to end by the year 2002.
Therefore, there has existed a need to develop processes for the formation of rigid polyurethane foams which utilize blowing agents having a zero ozone depletion potential and which still provide foams having low flammability, good thermal insulation properties and excellent dimensional stability.
A class of materials which have been investigated as such blowing agents include various hydrocarbons such as n-pentane, n-butane and cyclopentane. The use of such materials is well-known and disclosed, e.g., in U.S. Pat. Nos. 5,096,933, 5,444,101, 5,182,309, 5,367,000 and 5,387,618. Hydrocarbons offer many advantages such as zero ozone depletion potential, a very low global warming potential, low cost and are liquid at room temperature. One disadvantage of hydrocarbons however is their inherent flammability.
Rigid polyurethane foams used in building construction industry are closed cell in order to trap the blowing agent and benefit from its lower thermal conductivity, i.e., heat insulation capability. But presence of this trapped flammable gas in the cell presents a special challenge in terms of flammability performance of the closed celled foam. Though flammability of such foam has been a concern in general, surface burning characteristics of foam has been of particular concern.
Surface burning characteristics of materials are determined by test methods such as American Society of Testing Materials (ASTM) E 84 "Standard Test Method for Surface Burning Characteristics of Building Materials." It is used to assess the spread of flame on the surface of a material. Popularly known as "Tunnel test,"E84 exposes a 24 ft. long by 20 inches wide foam specimen to a controlled air flow and flaming fire exposure, adjusted so as to spread a flame along the entire length of a select grade oak specimen in 5.5 minutes. Generally the test is performed on core foam of chosen thickness but on occasion it is performed on faced products. Flame spread and smoke density are the two parameters measured in the test. The Flame Spread Index (FSI) takes into account both the rate and total distance of the propagation of a flame front, measured visually. The smoke factor is a time-integerated measurement of the occlusion of a visible beam of light. Material performance is put into categories namely 0-25 flame spread index is class I, 26-75 is class II, 76-225 is class III. Smoke limit of 450 is required in each of these classes. ASTM E 84 also has a number of other designations, such as Underwriters Laboratories 723, National Fire Prevention Association 255, or International Conference of Building Officials 8-1.
Since polyurethane foam laminates are used in building construction, it must adhere to the local building code requirement for flammability. When regulating materials, many of the model building codes [such as Building Officials and Code Administrators International Inc. (BOCA), International Conference of Building Officials (ICBO) and Southern Building Code Congress International Inc. (SBCCI)] and insurance rating organizations [such as Underwriters Laboratories (UL); Factory Mutual Research Corporation (FMRC)] refer to quality standards developed by standards-setting organizations such as ASTM. Generally the codes require that the foam core have a flame spread index of 75 or less and a smoke development rating of 450 or less, i.e., meet Class II rating in accordance to ASTM E 84. The rigid polyurethane based laminate boardstock used in the building insulation applications has exceeded this requirement and have historically been rated as Class I in the ASTM E-84 flammability test. Thus HCFC-141b blown foam currently used in the market place or CFC-11 blown foam used prior to 1993 phaseout of CFCs have been Class I.
A widely used method to improve the flammability performance of hydrocarbon blown closed cell rigid foam has been to add water to the formulation which when reacted with isocyanate releases carbon dioxide. This reduces the amount of hydrocarbon trapped in the closed cells of the foam. Adding water and thus reducing hydrocarbon has deleterious effects on foam's insulation properties and its structural properties, especially at low density. Use of water reduces the amount of low thermal conductivity gas (i.e. hydrocarbon as opposed to carbon dioxide) in the closed cells of the foam. This is not desirable as a key attribute of closed cell rigid polyurethane foam is its good insulation properties and good structural properties all at low densities.
Other attempts to improve the flammability performance in general and surface burning characteristics in particular of hydrocarbon blown closed cell rigid foam has centered around adding a halogenated blowing agent (e.g., U.S. Pat. No. 5,384,338; U.S. Pat. No. 5,385,952; U.S. Pat. No. 5,420,167, and U.S. Pat. No. 5,556,894) to the foam formulation. Such attempts have met with limited success. Known methods for producing foams using hydrocarbon as blowing agent and reaction systems used in such methods have not been found to produce rigid polyurethane foams having good flammability performance, in particular class I rating in ASTM E 84 test, and commercially attractive thermal and structural properties at densities which are sufficiently low to make their use feasible. In short, the flammability performance associated with such hydrocarbon blown foams have generally been inferior to CFC and HCFC blown foams.
Accordingly, there remains a need for a process for the production of closed celled rigid polyurethane or urethane-modified polyisocyanurate foam which utilizes hydrocarbon blowing agent with minor amount of water and which provides foams having good flammability performance, in particular class I rating in ASTM E 84 test.
It is an object of the present invention to provide closed celled rigid polyurethane or urethane-modified polyisocyanurate foams blown with hydrocarbons which have good thermal insulation and structural properties along with the improved fire properties.
It is another object of the present invention to provide closed celled rigid polyurethane or urethane-modified polyisocyanurate foams blown with hydrocarbon which provides the above fire, insulation and structural properties at low densities (comparable to those used for CFC or HCFC blown foam) using minimal amounts of halogens. Lower halogen levels enhance the environmental acceptability of the foam.